Yes, many types of flies bite. While the common housefly you see buzzing around your kitchen does not bite, dozens of other fly species feed on human blood. These biting flies include horse flies, deer flies, black flies, stable flies, sand flies, and tiny biting midges sometimes called “no-see-ums.” The bites range from mildly annoying to genuinely painful, and a few species can transmit diseases.
Which Flies Bite and Which Don’t
The common housefly (the one landing on your food) cannot bite. It has sponging mouthparts designed to soak up liquids, not pierce skin. But it has plenty of relatives that can and do bite people.
Horse flies and deer flies are the largest biters. These stocky flies belong to the family Tabanidae and are most active in warm months near water. Deer flies typically target your head and shoulders, while horse flies go for exposed skin anywhere on the body. Stable flies look almost identical to houseflies but have a sharp, forward-pointing mouthpart that gives them away. They primarily feed on livestock but will readily bite humans when animals aren’t available.
Black flies are small, dark, humpbacked flies that breed near running water. About 40 species exist in Colorado alone, and they’re found across North America. Biting midges are even smaller, only 1 to 3 millimeters long, earning their nickname “no-see-ums” because you often feel the bite before you spot the insect. Some midge species can travel up to 10 miles from their breeding site. Sand flies and snipe flies round out the list of common biters, though they tend to be more regionally concentrated.
In all biting fly species, only the females bite. They need blood to develop their eggs. Males feed on nectar and plant sap.
Why Fly Bites Hurt More Than Mosquito Bites
Mosquitoes use a needle-like mouthpart to slip into your skin with minimal damage. Biting flies take a completely different approach. Horse flies and deer flies have blade-like mouthparts that slash the skin open, then lap up the blood that pools in the wound. This is why you often see a small amount of bleeding at the bite site, something that rarely happens with mosquitoes. The cutting action also explains the sharp, immediate pain. You’ll usually know the instant a horse fly bites you.
Stable flies use a bayonet-style mouthpart that punctures rather than slashes, but it’s still far less delicate than a mosquito’s probe. Black flies and biting midges are smaller, so their bites cause less initial pain, but the allergic reaction that follows can produce intense itching and swelling that lasts for days.
What a Fly Bite Looks and Feels Like
The first sign is usually pain at the moment of the bite. After that, you’ll notice a raised, red or discolored bump that may be sensitive to touch. With horse fly and deer fly bites, there’s often a visible drop of blood at the center. Most fly bites heal on their own within a few days.
The severity of your reaction depends on the species and your individual sensitivity. Some people develop large, swollen welts from black fly bites that last a week or more, while others get only a small red mark. If you’ve had a large localized reaction to a fly bite in the past, you’re statistically more likely to have a stronger reaction next time. Delayed allergic reactions, sometimes called serum sickness, can begin several days after a bite and cause broader symptoms like hives. True anaphylaxis from fly bites is uncommon but possible, with symptoms like difficulty breathing or swelling away from the bite site.
Diseases Biting Flies Can Spread
Most fly bites in North America are painful nuisances, not health emergencies. But biting flies are capable of carrying pathogens. Deer flies in the western United States can transmit the bacterium that causes tularemia, a serious but treatable infection. Globally, the picture is more concerning. Black flies transmit river blindness, a parasitic disease affecting millions in sub-Saharan Africa and parts of Central and South America. Sand flies spread leishmaniasis, a parasitic disease found in tropical and subtropical regions. Tsetse flies in sub-Saharan Africa carry the parasite responsible for sleeping sickness. Biting midges in the genus Culicoides can transmit Oropouche fever, a viral illness found in South America.
If you’re traveling to tropical regions where these diseases are endemic, fly bite prevention becomes a genuine health priority rather than just a comfort issue.
How to Prevent Fly Bites
Insect repellents registered with the EPA work against biting flies. The most effective active ingredients are DEET, picaridin, IR3535, and oil of lemon eucalyptus. The CDC recommends DEET concentrations of 20% or higher for meaningful protection. Products with less than 10% active ingredient typically protect for only one to two hours. Going above 50% DEET doesn’t add any extra benefit. For children, products should contain no more than 30% DEET.
Beyond repellents, practical steps make a real difference. Horse flies and deer flies are most active during daylight hours, especially on warm, calm days near water. They’re attracted to dark colors and movement, so wearing light-colored, long-sleeved clothing helps. Black flies are worst in late spring and early summer near streams and rivers. Biting midges are most active at dawn and dusk and are small enough to pass through standard window screens, so finer mesh screens or staying indoors during peak hours offers better protection.
Treating a Fly Bite at Home
Clean the bite with soap and water as soon as you can. Resist the urge to scratch, since broken skin invites infection. An over-the-counter antihistamine can reduce itching, and hydrocortisone cream applied to the bite helps with both itch and swelling. A cold compress or ice pack wrapped in a cloth brings down inflammation and numbs the area. Most bites resolve within a few days with this basic care.
Watch for signs of infection: increasing redness that spreads outward from the bite, warmth, pus, or worsening pain after the first day or two. Symptoms that show up away from the bite site, like hives on other parts of your body, muscle spasms, or trouble breathing, signal an allergic reaction that needs prompt medical attention.